## Surface Circulation Study

of Waters Near Ochlockonee Bay, Florida

by

Peter Lazarevich

27 July 2007

Florida State University Department of Oceanography Tallahassee, Florida 32306-4320 U.S.A

### 1. Introduction

This report presents the design and fabri cation of a GPS/radio-tracking system for measuring surface circulation, and an analysis of the field data collected by our group during May and June, 2007 in the Ochlockonee Bay area of Northwest Florida. The purpose of this study was to observe circulation patterns that could explain the occurrence and timing of bacterial contamination at nearby beaches. Thes e sites are regularly monitored by the Florida Department of Health as part of their “Florida Healthy Beaches Program”, which was initiated in 1998. In particular, Mashes Sands Beach, Waku lla County, has been monitored since 2000 and

over that time there have been se veral advisories and warnings issued for contamination. At the present time, July 2007, Mashes Sands Beach has been under an advisory since mid-June. Our field work began prior to this advisory, and continued for several weeks afterwards.



### 2. Study Area

Ochlockonee Bay (Figure 1) is located within the Big Bend region of Northwest Florida, about 80 km south of Tallahassee, Florida. Freshw ater input comes primarily from two rivers on the western end, the Ochlockonee and Sopchoppy Rive rs, and it is open to Apalachee Bay on the eastern end. Ochlockonee Bay also has several tid al creeks and shallow marshes. Its basin is about 10 km long and 3 km wide, with depths ra nging from 3 m to 7 m in the channel and from

1 m to 2 m outside. There are extensive shoals throughout, and, at low tide, some become exposed. Mashes Sands Beach is located just outside of Ochlockonee Bay. A shoal extends some 3 km away from the beach. North of Mashes Sands, waters are also quite shallow. Broad marshes separate several, relatively closed-off bays. Dickson Bay, locat ed directly north of Mashes Sands Beach, is one example.

Figure 1. The study area.

### 3. Drifter

The original design goal was to make a dr ifter capable of meas uring currents over the course of several weeks, with GPS fixes recorded every hour or better and transmitted by radio

to a base station for real-time monitoring and recovery. This design relies upon three main components: a GPS receiver, radio transceiver, and a microprocessor controller, each available commercially as OEM components. Selection of the components was made based upon a compromise between battery efficiency, small-pr ofile, and light-weight. However, due to the short duration of this project, additional consid eration was given to se lecting components that could be integrated and deve loped with ease. In the end, the following components were selected: a Lassen iQ GPS receiver from Trimbl e, an AC4790-200 transceiver from Aerocomm, and a BS2pe Microprocessor from Parallax. Tabl e 1 lists the major drifter components and their specifications.

Component Manufacturer and model Specifications
GPS receiver Trimble, Lassen iQ 27 mA current draw (@ 3.3 V) NMEA output (GGPA string) Reacquisition time < 5 seconds

External antenna Radio transceiver Aerocomm, AC4790- 200 50 mA current draw (@ 3.3 V) 100 mW output power 4 mile range (advertised) Peer-to-peer protocol Frequency Hopping Spread Spectrum 902 – 928 MHz frequency band 76.8 Kbs data rate (baud rate) External half-wave dipole antenna Microprocessor Parallax, BS2pe 15 mA cu rrent draw in run mode (@ 5 V) 40 uA current draw in sleep mode Programmable in BASIC 32 Kbyte memory I/O lines for component control Battery AA Portable Power, 18650 2000 mAh each Li-Ion, rechargeable Voltage Regulator Maxim, MAX883 5 V regulated output 200 mA output limit

Table 1. Parts list of the drifter – major components, their manufacturer and model, and specifications.Twelve drifters and one base station were built, as shown in Figure 2. The drifters were programmed to acquire a GPS fix every two minutes , store the fix into memory, and then wake up at selected times and broadc ast a radio message that include d the last GPS fix made. The base station was programmed to constantly li sten for these transmissions and output the messages via RS-232 for display on a computer.

Figure 2. The twelve drifters and base station, being prepped prior to a deployment.

The drifters were designed to be compact and low-profile in order to avoid being seen on the water by boaters. Figure 3 shows what an (unpa inted) drifter looked like in the water. It was barely visible at 100 m. During field tests, it was found that the drifter was quite stable in the water, and that both the GPS and radio antenn as remained slightly above the water line.

Figure 3. View of the drifter (center of the photo) in the water from a distance of 100 m, plus a close-up zoom.

### 4. Observations

A total of nine deployments were made: fi ve in Ochlockonee Bay, two in Dickson Bay, one in Ochlockonee River, and one in Apalachee Bay. The first deployment was a short test of the drifter system. The second, th ird, and fourth deployments were made at the same time that water samples were collected by the University of South Florida (USF) group. The remaining deployments were longer in duration and exhibite d interesting circulati on patterns. The table below summarizes the deployments.

Deployment Date Location Duration Number of Drifters

Number (2007) (hours) Deployed Recovered

1 May 8 Ochlockonee Bay 0.67 2 2

2 May 9 Ochlockonee River 1.00 3 3

3 June 6 Ochlockonee Bay 1.25 8 8

4 June 6 Ochlockonee Bay 0.92 8 8

5 June 7 Dickson Bay 2.25 8 8

6 June 25 Dickson Bay 10.83 12 11

7 June 27 Ochlockonee Bay 22.92 10 6

8 June 29 Apalachee Bay 3.67 6 6

9 June 30 Ochlockonee Bay 6.92 6 6

Table 2. Listing of the drifter deployments.

In the figures that follow, drifter tracks fr om the nine deployment s are plotted on maps showing the deployment and recovery locati ons, and 5-minute interpolated positions. Bathymetry contours and a recent ae rial photo of the area are added. The aerial helps to identify submerged shoals, which show up as a lighter colo r than the deeper wate r. In addition, tidal information is also presented, as the circulation is dominated by the tides. The tidal phase is represented by a color-coding of the interpolated positions. Red dots indicate an incoming tide and blue dots indicate an outgoing tide.4.1. 

### Deployment #1, 08 May 2007

The first deployment was a short test of the drifter system, just inside the mouth of Ochlockonee Bay. Two drifters were deployed simultaneously, about 20 m apart, at a channel marker which helped to serve as a visual referen ce. They were kept in sight at all times, and

radio contact was maintained th roughout the trial. An outgoi ng tide carried th e drifters out towards the mouth of Ochlockonee Bay. During th e 40 minutes in the water, there was no appreciable relative motion between the drifters.

Figure 4. Drifter tracks for deployment #1, 08 May 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide. The red dots with black bo rder are the locations where the USF team collected water samples.4.2. 

### Deployment #2, 09 May 2007

The second deployment was at a USF water sampling site, in the Ochlockonee River. Three drifters were deployed simultaneously, about 20 m apart. They were in the water for 60 minutes, and in that time one of the drifters washed up along the river’s southern shore. While there was an outgoing tide in Ochlockonee Bay, the drifters traveled upriver, most likely due to a substantial phase lag in the tidal forcing at this location.

Figure 5. Drifter tracks for deployment #2, 09 May 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide. The red dot with black border is the location where the USF team collected a water sample.4.3. 

### Deployment #3, 06 June 2007

The third deployment was near USF water sampling sites, offshore of Mashes Sands Beach. Eight drifters were deployed simulta neously, in two groups, about 50 m apart within each group. An outgoing tide carried the drifte rs out towards the mouth of Ochlockonee Bay. They were in the water for 75 minutes, and some relative motion was observed.

Figure 6. Drifter tracks for deployment #3, 06 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide. The red dots with black bo rder are the locations where the USF team collected water samples.4.4. 

### Deployment #4, 06 June 2007

Later, on June 6, another deployment was made nearby Mashes Sands Beach. Eight drifters were deployed simultaneous ly, about 50 m apart. They we re in the water for 55 minutes, carried out on an outgoing tide. At recover y, two drifters had grounded along the northern shore of Ochlockonee Bay, three were still tracking currents with Ochlockonee Bay, and the other

three had traveled north around Ochlockonee Point. This deployment clearly demonstrates how

varied currents can be around a coastline and over shoals.

Figure 7. Drifter tracks for deployment #4, 06 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide. The red dots with black bo rder are the locations where the USF team collected water samples.4.5. 

### Deployment #5, 07 June 2007

The fifth deployment was within Dickson Ba y, north of Mashes Sands Beach. Eight drifters were deployed simultaneously, approximate ly 100 m apart. Altho ugh the predicted tides indicated an outgoing tide, the drifters moved north, in appa rent opposition to the tides. However, they were traveling towa rds deeper waters in the channel to the north. This could be how waters in Dickson Bay predominately flow during an outgoing tide, instead of to the southeast over the shoals. They were in the water for about 2 hours and 15 minutes

Figure 8. Drifter tracks for deployment #5, 07 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide.4.6. 

### Deployment #6, 25 June 2007

The sixth deployment was outside Dickson Ba y. All twelve drifters were deployed simultaneously, about 200 m apart. They were deployed on the incoming tide and were carried a distance of about 4 km back into Dickson Bay. Remarkably, the tracks converged, indicating that the flow in Dickson Bay was primarily dire cted in towards the channel. The drifters remained relatively close together, almost for the duration of the incoming tide; some separation was observed as several became grounded or were trapped around shoals. Later, when the tides reversed, the drifter separation ge nerally increased. At recover y, six drifters had grounded along the coast, two were nearly gr ounded on shoals, three were tracking currents, and one was not

heard from and therefore lost. This deployment lasted 10 hours and 50 minutes.

Figure 9. Drifter tracks for deployment #6, 25 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, blue for an outgoing tide and red for an incoming tide.4.7. 

### Deployment #7, 27 June 2007

The seventh deployment was made at the mout h of Ochlockonee Bay. Ten drifters were deployed in sequential pairs, approximately 2 m ap art in distance and 10 minutes in time. They were deployed on an outgoing tide and were in itially carried towards Ochlockonee Bay. They then turned to the southeast. By the end of the outgoing tide, the drif ters were approximately 1.5 km away from the deployment site. Around this time some drifters show a break in their tracks. The most likely explana tion is the drifters encountered an exposed shoal and turned onto their sides, submerging the GPS antenna and inte rrupting all drifter activity. Later, when the tides reversed, the floats came off the shoal and were carried into Ochlockonee Bay. Unfortunately, four of the drifters were lost. A nother four drifters failed toward the end of their missions, but were recovered by ch ance, close to the two drifters that continued working until they were located 10 km inside Ochlockonee Bay. In the figure, th e cyan stars east of the bridge represent the location of last valid GPS for th e four failed units. This was the longest deployment, lasting 22 hours and 55 minutes.

Figure 10. Drifter tracks for deployment #7, 27 June 2007. Deployment sites are indicated with the green stars. Here, cyan stars indicate either the recovery site or location of last valid GPS fix. The white lines indicate the drifter trajectories. Dots represent the 5-minute interpolated positions, blue for an outgoing tide and red for an incoming

tide. The red dots with black border are the loca tions where the USF team collected water samples.4.8. 

### Deployment #8, 29 June 2007

The eighth deployment was well outside Dick son Bay, in Apalachee Bay. Six drifters were deployed in sequen tial pairs, approximately 2 m apart in distance and 15 minutes in time. An outgoing tide carried the drifte rs out towards the southeast. They were deployed for 3 hours and 40 minutes, and no significant relative moti on was observed. The trial was ended early due to increasingly rough waves.

Figure 11. Drifter tracks for deployment #8, 29 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, and are all blue for an outgoing tide. The red dots with black bo rder are the locations where the USF team collected water samples.4.9. 

### Deployment #9, 30 June 2007

The final deployment was made in the cente r of Ochlockonee Bay. Six drifters were deployed in sequential pairs, approximately 2 m ap art in distance and 10 minutes in time. They were released on an outgoing tide and travel ed towards the southeast, until they became temporarily grounded on an exposed shoal. Later, when the tides reversed, the floats came off the shoal. Two pairs then turned northward and were carried about 4 km back inside Ochlockonee Bay while the other pair continued to the southeast and eventual ly were carried into a tidal creek, Chaires Creek. The trial lasted for 6 hours and 55 minutes. Apart from the separation caused by the exposed shoal, the drif ter pairs remained remarkably close together.

Figure 12. Drifter tracks for deployment #9, 30 June 2007. Deployment and recovery sites are indicated with the green and cyan stars, respectively. The white lines indicate the drifter traj ectories. Dots represent the 5-minute interpolated positions, blue for an outgoing tide and red for an incoming tide. The red dots with black border are the locations where the USF team collected water samples.

### 5. Analysis

Tidal information was obtained from the NO AA predicted tide tables, location on the internet at: http:// tidesandcurrents.noaa.gov/. Although pr edicted tides for Ochlockonee Bay were not available, there were data for nearby locations- Turkey Point and Shell Point. These two sites are roughly equal distance from Ochloc konee Bay, and the average of the two was used to determine predicted tides for the study area. The following three figures show the predicted tides for Ochlockonee Bay during the time of the deployments.

Figure 13. Ochlockonee Bay tides (red line) for deployments #1 and #2 (blue dots).

Figure 14. Ochlockonee Bay tides (red line) for deployments #3 through #5 (blue dots).Figure 15. Ochlockonee tides Bay (red line) for deployments #6 through #9 (blue dots).

Drifter velocities were calculated for all dr ifter tracks. Figure 16 shows the histogram of all velocities, computed at all 5-minuted inte rpolated positions. The sample had a total of 2865 velocities, with a mean of 20.0 cm/s.

Figure 16. Histogram of all drifter velocities.

### 6. Conclusions

Overall, the GPS/radio drifter system perf ormed quite well. The GPS unit provided accurate positioning at a relatively low-power cost. The radio performed below expectations, however, as the effective range was only 0.5 km. This was primarily due to the transmitting baud rate (76.8 Kbs) and the Frequency Hopping Spread Spectrum operating mode. While these capabilities are desired for large data throughput and data security , they require a higher signalto-noise ratio for successful transmissions. A future version of the drifter would include a

different transceiver in order to achieve the desired range.

Other changes to the design would include a new microcontroller th at operated on 3.3 V

instead of the 5 V the current de sign implements. This would simplify the design and improve battery efficiency. Also, a short, wide housing would enable the drifter to operate while on a shoal, instead of having the GPS antenna submerge as it has been found to do. Table 3 summarizes the performance of th e current version of the drifter and suggests what is possible with a future design.

Specification Current version Future version
Battery lifetime 3 days 15 days Number of GPS fixes 1790 10,000 Range 0.5 km 5 km Weight 0.61 kg 0.5 kg Height 26 cm (46 cm with antenna) 10 cm (20 cm with antenna)

Width 7 cm 18 cm

Table 3. Drifter performance.



### 7. Acknowledgements

I would like to thank my stepfather, Bill Coch ran, for his technical guidance in the drifter design. The project greatly benefited from his pr actical approach towards hardware development and software programming. The Florida State University Coastal and Marine Laboratory provided the boats needed to carry out the fiel d work. Stephanie Fahrny, Mike Lavender, and Joel Valdez (Florida State University) assisted in the recovery of the lost drifters in deployment #7. Funding for this project was pr ovided by the Florida Department of Health, grant #091004-523-020947.